Rational Design of Ruddlesden–Popper Perovskite Ferrites as Air Electrode for Highly Active and Durable Reversible Protonic Ceramic Cells

Highlights A novel A/B-sites co-substitution strategy was introduced to enhance the performance and durability of Ruddlesden–Popper perovskite Sr3Fe2O7−δ (SF)-based air electrodes for reversible protonic ceramic cells (RePCCs). Simultaneous Sr-deficiency and Nb-substitution in SF result in Sr2.8Fe1.8Nb0.2O7−δ (D-SFN), offering improved structural stability under RePCC conditions by suppressing the formation of Sr3Fe2(OH)12 phase. The introduction of Sr-deficiency enhances oxygen vacancy concentration in D-SFN, promoting efficient oxygen transport within the material and contributing to excellent activity in RePCCs. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01397-2.

In this experimental titration, 0.1g of the powder sample was completely dissolved using a concentrated HCl solution (6 mol L -1 ), along with excess KI powder.Subsequently, the resulting solution was diluted with water to maintain a weakly acidic pH (pH=6).The solution was titrated with fresh standard Na2S2O3 solution (0.05 mol L -1 ) twice, and near the titration endpoint, a fresh starch indicator was introduced to indicate the endpoint.The average volume of consumed Na2S2O3 solution was calculated from the two titration experiments to determine the valence state of the Fe element and the concentration of oxygen vacancies.

S1.2 Electrical Conductivity Test
The electrical conductivity of the S3-yFNx was examined by the DC four-probe method [S1, S2].Dense bar samples were prepared with silver wires affixed as current collectors.A constant current of 100 mA was applied to the sample, while the voltage was recorded with a digital source meter (Keithley 2440).The conductivity (σ) of the material was calculated using the following Eqn.(S1): where ρ is the resistivity, Ω m, I and U are the output current signal, A; U is the output voltage signals in A and V, respectively, and  2 ,  1 , and  3 are the length, width, and thickness of the bar sample in cm.
The electrical conductivity relaxation (ECR) test was done using the same configuration.The sample was placed in a 21% O2-71% N2 atmosphere.Once the output signal was stabilized, the atmosphere was quickly switched to 10% O2-90% N2 until it was stabilized again.The corresponding relaxation curves were recorded and converted into normalized conductivity (g(t)) using the expression (S2): where  0 ,   , and  ∞ represent the conductivity at t=0, t=t, and t=∞ in S cm -1 . ℎ represents the chemical bulk diffusion coefficient of the material (cm 2 s -1 ), while  1 ,  2 ,  3 denote the nth, mth, and pth roots of the transcendental equation, respectively.Further details of the fitting procedure can be found in the literature [S3, S4].In this study, the relaxation curves were fitted using the ECRTOOLS tool developed by Prof. Ciucci et al. [S5].
Ni-BZCYYb | BZCYYb | S3-yFNx single cells were fabricated by a co-pressing method.NiO (standard, FuelCellMaterials.Co.), BZCYYb powder, and starch were thoroughly mixed and pressed as the fuel electrode Ni-BZCYYb, in which the starch functions as the pore former.Subsequently, the pure BZCYYb electrolyte powder was evenly spread and pressed onto the surface of the NiO-BZCYYb, calcined at 1450 o C for 5 h to form a Ni-BZCYYb| BZCYYb half-cell.Afterward, the S3-yFNx slurry was sprayed on BZCYYb's surface and calcined at 1100 o C for 2 h to produce the single cell.

S1.4 EIS Data Analysis
The Distribution of Relaxation Time (DRT) Method facilitates the extraction of substep information about the electrode reaction.The relevant data processing was performed utilizing DRTTOOLS, which was developed by Prof. Ciucci et al. [S6,S7].
DRTTOOLS is specifically designed for EIS data analysis, allowing for the extraction of electrochemical responses at various frequencies.This approach capitalizes on the characteristic time distributions inherent to distinct physicochemical processes.By deconvoluting EIS data into these characteristic distributions based on time scales, the electrode reactions can be effectively identified [S8, S9].

S1.5 Computational Details
Oxygen Reduction Reaction (ORR) proceeds through a four-electron mechanism The Gibbs free energy (∆G) change for ORR/OER intermediates was determined using the equation ( S7) [S12]: Here, ∆E represents the energy change in each step, ∆ZPE is the change in the zeropoint energy calculated from vibrational frequencies, ΔS is the entropy change based on thermodynamics databases, and T denotes the room temperature of 298.15 K. − is the standard electrochemical potential of the electron involved in the reaction, obtained when the electrode potential is referenced to the reversible hydrogen electrode.
Simultaneously, the standard electrochemical potential of the proton (  2 ) is set to be equivalent to that of a hydrogen atom in gaseous H2 ( 2 ).Owing to limitations in characterizing the triplet state of the O2 molecule within the present DFT framework, the free energy of the O2 molecule was determined by the following equation [S13]: Corrected free energy values were computed using a plugin for VASP, named VASPKIT.
To quantitatively assess the stability of the perovskite lattice, the formation energy of the Ruddlesden-Popper layered perovskite S3-yFNx crystal structure was calculated using the formula below [S14, S15]: The formation energy is determined by subtracting the energy of the reactants on the left-hand side from the energy of the products on the right-hand side of the equation.
The more negative the formation energy, the higher the stability of the perovskite.
Fig. S1 Refined XRD pattern of S3-yFNx materials synthesized by sol-gel method

Table S1
Lattice parameters of S3-yFNx powders obtained by XRD Rietveld refinement

Table S2
Rietveld refinement results derived from the XRD pattern of as-synthesized SF

Table S3
Rietveld refinement results derived from the XRD pattern of as-synthesized SFN

Table S6
Dchem and kchem values for SF, SFN, and D-SFN samples at various temperatures

Table S9
Performance comparison of reported proton-conducting electrolysis cells at an applied voltage of 1.3 V